How to convert Java "Arrays.copyOfRange" function to C#?

Question

Working on MergeSort in Java:

public void mergeSort(int[] A)
{
     if (A.length > 1)
     {
         int q = A.length/2;
         int[] leftArray = Arrays.copyOfRange(A, 0, q);
         int[] rightArray = Arrays.copyOfRange(A,q,A.length);
         mergeSort(leftArray);
         mergeSort(rightArray);
         merge(A,leftArray,rightArray);
     }
}

The recursion in the code above works well in Java.

So for curiosity I want to convert function Arrays.copyOfRange from java to c#. Array.copy in C# takes five arguments. Do you know any simpler function in c# do get certain elements of the array starting at position x to y (like java).

In c# I coded the above method like this:

public void mergeSort(int[] A)
{
    if (A.Length > 1)
    {
        int q = A.Length / 2;
        int[] leftArray = new int[q];
        int[] rightArray = new int[A.Length];
        for (int i = 0; i < q; i++)
        {
            leftArray[i] = A[i];
            Console.WriteLine(leftArray[i]);
        }

        for (int i = q; i < A.Length; i++)
        {
            rightArray[i] = A[i];
            Console.WriteLine(rightArray[i]);
        }
        Console.ReadKey();

        mergeSort(leftArray);
        mergeSort(rightArray);
        merge(A, leftArray, rightArray);
    }
}

As you can see I have replaced Arrays.copyOfRange functions in Java with two loops in c# and this works in c# without recursion. However calling mergeSort(leftArray) and mergeSort(rightArray) it prints this in c#:

Process is terminated due to StackOverflowException!!

Any better idea on how to get certain elements in c#?

Why is this question downvoted? This is precisely the info I was looking for — Mishax
– Mishax, Commented Sep 12, 2014 at 8:50
I also am baffled by the downvotes. Perhaps the downvoters are not familiar enough with both java and C# to grasp that the java function copyOfRange has no direct equivalent in C#, in the sense that C# requires a separate action to create an array, before performing Array.Copy. This is why a newcomer to C# may struggle a bit before seeing a good equivalent. IMHO this question is very useful to some beginners. — ToolmakerSteve
– ToolmakerSteve, Commented Sep 23, 2015 at 8:51

user2864740 · Accepted Answer · 2014-01-21 22:13:48Z

12

The problem is the ported array copy is not doing the same thing.

Given an input of [a,b,c,d,e,f] the Java code is creating two arrays, [a,b,c] and [d,e,f] while the C# port is creating two arrays, [a,b,c] and [0,0,0,d,e,f]. Notably,

The new right array is the size of A (new int[A.Length]). This is what causes the StackOverflowException as the termination case is never reached, and;
The new right array is only assigned values starting at the q index, which is the half-way index.

Consider this replacement method using Array.Copy - a method with the same signature can be used as a drop in replacement in ported code, as long as what's inside the method has the same effect as the original.

int[] copyOfRange (int[] src, int start, int end) {
    int len = end - start;
    int[] dest = new int[len];
    Array.Copy(src, start, dest, 0, len);
    return dest;
}

Or, a version that does it with a loop but without the issue in the original port. Another reason to use discreet functions - it makes the task easy to see and reason about. Being able to also eliminate duplicate code doesn't hurt.

int[] copyOfRange (int[] src, int start, int end) {
    int len = end - start;
    int[] dest = new int[len];
    // note i is always from 0
    for (int i = 0; i < len; i++)
    {
        dest[i] = src[start + i]; // so 0..n = 0+x..n+x
    }
    return dest;
}

If you're lazy like me, the code could also be trivially written using LINQ.

int[] leftArray = A.Take(q).ToArray();
int[] rightArray = A.Skip(q).ToArray();

edited Jan 21, 2014 at 22:13

answered Jan 21, 2014 at 21:48

user2864740

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5 Comments

Milot25 Over a year ago

calling your first method: int[] copyOfRange (int[] src, int start, int len) with parameters: A, 0, q prints the following error: Source array was not long enough. Check srcIndex and length, and the array's lower bounds.

user2864740 Over a year ago

@Milot25 That was my original version, but (due to an oversight) it didn't match the Arrays.copyOfRange contract - oops. I've updated the code in the post for both functions to take in an end parameter (from which len is derived) so that can actually be used as a drop-in replacement for Java's Arrays.copyOfRange.

Milot25 Over a year ago

Your function is working. However I think the recursive method is causing something wrong in C#. Given an array: int[] a = {6, 3, 8, 5, 1, 9, 7, 3, 2, 4}; it prints only 3,6 whereas in Java: 1,2,3,4,5,6,7,8,9

user2864740 Over a year ago

@Milot25 Maybe the merge method is wrong? The input array and the output array must be the same size - if not, then elements were lost. I am very suspicious of this reported result as the final output array is the same as the input array (and thus even if there are bunch of 0 elements, it should be the same length).

Milot25 Over a year ago

I realize I have made some semantic mistake in merge algorithm implementation. Now everything works fine. Thanks very very much Sir.

user16665136 · Accepted Answer · 2022-04-15 15:06:41Z

0

Now in modern C# there is a simple solution for this:

string[] cuttedArray = array[2..6] // from second element to sixth element exclusive

answered Apr 15, 2022 at 15:06

user16665136

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